Conventional methods of manufacture for bonded assemblies such as support structures, casings, discs and drums for gas turbine engines often result in low “fly to buy” ratios (i.e. result in large amounts of wasted material) and require significant amounts of machining to achieve the finished component. Consequently, such conventional methods are expensive both in terms of raw materials and labour. Near net shape manufacture can offer greatly improved fly to buy ratios and large component cost reductions from greatly improved material utilisation rates and reductions in finishing operations such as machining, which may improve factory capacity and throughput.
Hot Isostatic Pressing (HIP) diffusion bonding is one manufacturing method which is suitable for producing high integrity and low cost near net shape components. HIP bonding involves placing powder or one or more part sintered parts into an airtight container and then applying heat and pressure to an external side of the container. The container comprises a malleable metal such as mild steel or stainless steel. One of the main factors affecting the efficacy and cost of this process is the use of the container. The integrity of the container is critical to the success of the HIP diffusion bonding process, since any leaks in the container will allow gas into the powder, which will therefore fail to consolidate to form a bonded component. The complexity of the canister is one of the factors that limit the application of HIP diffusion bonding, since for complex assemblies an extremely complex and potentially costly container would be required. In addition the container material is too weak to support the assembly at the high temperatures required for HIP, and therefore additional tooling would be required to maintain the dimensional conformance of the assembly during sintering. In some case, due to the high temperatures required, the tooling may comprise expensive refractory metals or ceramics.
Consequently, it is desirable to avoid the use of the container in HIP diffusion bonding.
U.S. Pat. No. 5,593,085 discloses one prior method of diffusion bonding in which part sintered components are used to form a container surrounding a cavity. The cavity is evacuated, sealed, and heat and pressure are then applied to the components to collapse the cavity, and thereby bond the components together. However, in order to evacuate the cavity, a sealing tube is required, which must itself be inserted, crimped and sealed prior to evacuation of the cavity. This method therefore requires additional process steps, and is therefore relatively slow and expensive.
GB 2257385 discloses a further prior method of diffusion bonding. First and second components are abutted against one another to form a cavity therebetween. The cavity is then partly sealed using tungsten inert gas (TIG) welding, and the cavity is then evacuated. The cavity is then completely sealed under vacuum using electron beam welding. Heat and pressure are then applied to the external surface of the cavity to thereby collapse the cavity and form a diffusion bonded article. However, such an arrangement is not suitable for components having complex geometries, since TIG welding requires the welding to be conducted in an inert gas atmosphere, such as an argon gas atmosphere.
The present invention describes a method of forming a bonded assembly and a bonded assembly formed by the method which seeks to overcome some or all of the above problems.